Party lines provide a means of servicing a plurality of subscribers using only a single communication line wire pair known as a "subscriber loop" originating at the Central Office (C.O) or a PBX.
The telecommunication system as well as those persons using it regard each subscriber as a separate entity. Thus, it is desirable to have the capability to notify one subscriber that it is receiving an incoming call without simultaneously and falsely alerting the other subscribers.
A party line subscriber alerting system is characterized by its capability to perform full-selective, semi-selective and coded ringing alerting functions.
A system performs full-selective alerting when each subscriber associated with the party line is alerted only when he receives an incoming call over that line. In semi-selective alerting, the subscribers associated with a party line are divided into groups of two and both subscribers of a group are alerted whenever any one of them receives an incoming call. Finally, coded ringing is performed by using a different number of rings of a different type ringer signal to alert each subscriber, or alternatively, each group of subscribers.
A variety of methods may be used to implement party line ringing. The most common scheme used to selectively alert a party line subscriber employs a DC biasing voltage in conjunction with grounded wiring of the alerting mechanism and superimposes a 20 Hz signal upon this potential. Telecommunications Transmission Engineering, Vol. 2, (Winston-Salem, Western Electric Inc. Tech. Public. 1977). Another selective alerting scheme uses a different frequency ringer signal generated by the C.O. to alert each party of subscribers. This method is not commonly in use.
The simplest of the methods using DC biased and superimposed AC signaling is known as divided code ringing. This method involves the application of a ringer signal having a-48 V DC component and a 20 Hz AC component to either the TIP line or the RING line of a telephone line wire pair. This method is capable of performing two-party full selective ringing with only one party being alerted by a ringer signal on TIP and only the other being alerted by a ringer signal on RING. It also supports four party semi-selective ringing, and coded ringing for more than four parties.
A more sophisticated method known as superimposed ringing uses a ringer signal having either a positive or negative polarity 36 to 40 V DC component applied to either the TIP or RING wire of a party line. Systems utilizing this alerting method are thus capable of four-party full selective and eight party semi-selective ringing, and use coded ringing selectively to alert more than eight parties.
A system employing either method references signals provided by the Central Office or PBX over the TIP and RING lines of the communication line wire pair to a ground potential, which is ordinarily provided over the sheath of said wire pair.